β-Type zeolite is a zeolite having 12-membered-ring micropores and disclosed first in patent document 1, and is in extensive use as adsorbents and catalysts. However, there are the cases where when the zeolite is used at high temperatures, the catalytic performance decreases with collapse of the zeolite crystal structure. There is hence a need for improvement in heat resistance.
Known techniques for improving heat resistance include a method in which the molar ratio of SiO2/Al2O3 is heightened (patent document 2), a method in which a zeolite is treated at a high temperature (patent document 3), a method in which the diameter of crystallites is increased (patent documents 4 to 6), and a method in which fluorine is used (patent document 7).
For example, SCR catalysts employing a zeolite (catalysts based on NOx reduction reaction using ammonia as a reducing agent; “SCR” is an abbreviation for “selective catalytic reduction”) are required to have high low-temperature activity after a hydrothermal durability treatment, in particular, high activity at 300° C. and lower temperatures (patent document 8). Since SCR catalysts necessitate a large acid amount (low SiO2/Al2O3 molar ratio), neither the method in which the molar ratio of SiO2/Al2O3 is heightened in order to improve heat resistance nor a high-temperature treatment for inducing aluminum release from the framework can be utilized.
There is hence a desire for a zeolite which has a low SiO2/Al2O3 molar ratio and has high heat resistance and high catalytic activity, in particular, β-type zeolite which, when used as an SCR catalyst, shows a high NOx reduction percentage after a hydrothermal durability treatment even at temperatures as low as 200-250° C.
A process for producing β-type zeolite has been disclosed in which a tertiary alkanolamine is added for synthesis besides tetraethylammonium cations to thereby synthesize β-type zeolite having a crystal size as large as 0.1-3 μm (patent document 9). However, the β-type zeolite produced by this method did not have excellent hydrothermal durability.
Also disclosed is a method of synthesizing β-type zeolite in which a structure-directing agent (referred to as SDA) including diethylenetetramine is used (patent document 10). The β-type zeolite obtained by this method had a small primary-crystal diameter, and this zeolite also had insufficient hydrothermal durability.